The present applicant previously filed applications (Patent Document 1, etc.) with the invention for executing a task selected from tasks (which may be singular or plural) structuring a task combination.    Patent Document 1: Japanese Patent Application No. 2003-122139    Patent Document 2: Japanese Patent Application Laid-Open Publication No. 7-182183
In the preceding applications, the task combination is switched over after waiting for a completion of an executable task in the tasks structuring the task combination. Quick switchover cannot be attained by effecting the switchover after waiting for the completion of the task. Further, there might be presumed a case where the task must not be executed depending on a switchover factor as in the case of desiring to immediately interrupt the task before being switched over.